In the waveform multiplexing optical transmission system, a device is indispensable which splits plural wavelength multiplexed signal light beams into separate wavelength light beams, or which combines plural signal light beams into a sole wave guide path. As a device having such functions, a device employing an array waveguide lattice, referred to hereinafter as array wave guide (AWG), is felt to be promising. As an example of the AWG, a planar structure of an AWG disclosed in Takahashi et al, Meeting Abstracts to Autummn Meeting of the Society of Electronic Information Communication, Vol.4, page 272, is shown in FIG. 13.
In this AWG element, shown in FIG. 13, a quartz based light guide path is formed on a waveguide substrate, formed by a Si substrate. There are provided 11 input light waveguide paths 52, an inputside star coupler 53, as a recessed slab waveguide path, an array waveguide lattice 54, an output side star coupler 55, and an output waveguide path 56. Plural signal light beams of different wavelengths, entering one of the 11 input light waveguide paths 52, are subjected to phase shift determined by the wavelength by the array waveguide lattice 54 so as to be output at different output ports. Thus, it is possible to split the wavelength multiplexed signal light beams.
Takahashi et al manufactured a wave synthesizer/splitter, using 41 array waveguide lattices, in a 1.5 .mu.m wavelength band, with a frequency interval of 10 GHz and 11 channels, to achieve characteristics with -14 dB crosstalk, an insertion loss of 8 dB and a 3 dB transmission bandwidth of 6.5 GHz. The specific refractive index difference is 75%, with a substrate size being 4 cm by 56 cm. Meanwhile, a similar AWG device is disclosed in, for example, JP Patent Kokoku JP-B-7-117612.